A target peptide included in an albumin-containing sample, such as a disease marker, forms a complex with albumin in some cases. As a method of liberating the peptide from albumin, for example, the method described in US 2012/027740 is suggested.
In the method described in US 2012/027740, a sample containing a complex of a peptide and albumin is heated at a predetermined temperature to allow albumin to self-associate, thereby liberating the peptide from albumin.